The present invention relates generally to the emission of electrons from nano-structured materials and more particularly to the controlled emission of electrons from a cold electron source using nano-structures as the emitters.
It is a long strived for goal of various applications to use a cold electron source that can be modulated by low applied voltage, is highly uniform, addressable with high resolution, capable of generating high current density and has a collimated beam and long lifetime.
A cold electron source operates in a vacuum environment, where an electrical field is applied to an emitter to generate electrons. The emitter is connected to a negative cathode electrode. A positive gate or anode electrode extracts electrons from the emitter through a vacuum gap. In order for emission to occur, a field on the order of 5×109 V/m is required. A common scheme to induce emission with a reduced external field is to use a low work function material for the emitter and to enhance the local field at the emitter by sharpening it to a high aspect ratio. The higher the aspect ratio is, the lower the threshold field, generated from the electrodes, is for an emitter to emit electrons.
With a given emitter, the closer the gate is to the cathode, the lower the voltage that is required to modulate the emission. In order to achieve a modulation voltage of 10 V, the distance has to be close to one micrometer. Only a gate that is integrated onto the substrate by micro-fabrication can meet this requirement. And only this type of integrally gated electron source that is suitable for many applications and can achieve reasonable energy efficiency and be modulated by inexpensive and small COMS drivers.
The widest known field emission electron source is the Spindt type field emitter, where a conical or pyramid micro-tip of high aspect ratio is used as an emitter. The high cost and low yield associated with its fabrication process, and the high susceptibility of the tip to contamination and ion bombardment have effectively prevented this technology from wide application.
Recently, various nano-structures, such as nano-tubes and nano-wires, have been successfully synthesized. They are typically a few tens of nanometer in diameter and a few micrometers in length, thus a high aspect ratio. One nano-structure that has emerged as the leading candidate for field emitter is carbon nano-tube (CNT). Its extraordinary high electrical and thermal conductivity as well as strong chemical and mechanical stability make it an ideal field emitter. Numerous studies have reported its advantageous emission properties.
There has been great progress achieved in the growth of nano-structures, particularly CNT. It can be grown by various processes with transition metal or compound as catalyst. Through a combined scheme of selective passivation and catalyst application, one can control precisely where the CNT grows. The growth of CNT can be random in direction or aligned vertically to the surface. The spacing between aligned CNT and the tube diameter are largely determined and can be controlled by the catalytic particle spacing, size, and catalyst film thickness. Highly oriented and regularly spaced nano-tube or nano-wire arrays can also be grown on a template surface with pre-etched nano-pores. In summary, the growth of nano-structures, particularly CNT, has reached a high degree of controllability.
Despite the advantageous field emission properties of a single CNT and the ease of growing CNTs in abundance, a densely populated CNT forest can hardly emit any electrons, dashing the hope that such a CNT array would make a practical electron source with high emission site and current densities over large areas. The result has been attributed to the electrostatic screen effect between CNTs. The presence of neighboring CNTs reduces the field penetration down into the CNT film and counter acts the field enhancement factor derived from the high aspect ratio at the CNT tips. Even for a growth with a relatively lower density, where electrostatic effect is negligible, there is an ever-existing variation between CNTs in their height and apex. Those that have a higher aspect ratio and are closer to the extracting electrode experience a higher local field and always emit first, preventing the adjacent ones to do the same.
As a result, the hope of deriving an electron source of high emission site density from a vertically aligned CNT array has largely unfulfilled. Alternatively, a randomly oriented CNT film has provided a better solution. Emission is believed to result from those tips that protrude from the surface. There are two ways to apply these random oriented CNTs to a surface. One is to grow them on the surface. Two major problems plague these random CNT films. First, emission is inhomogeneous and often dominated by strong sites so that the emission site density is still relatively low despite of improvement over vertically aligned CNTs. Second, it is difficult to fabricate a gate electrode on top of these films without degrading their emission properties. The emitter cannot, therefore, be modulated with low voltage even though the film has a low threshold field. A conductive grid is often mechanically mounted on top of these films to modulate electron emission. A typical distance from the grid to the emitting film is 100 pm. Such a large distance results in high driving voltage, and the emission from underneath the grid goes directly to the grid without contributing to the output current. The difficulty in accurately and uniformly mounting and maintaining a fine grid over large area with a small gap is another hurdle. The second way to apply random CNTs to a surface is to mix pre-fabricated CNT with a conductive paste and then screen print them onto a surface, or mix the CNT with an organic resin and spin the mixture onto the surface. This approach has been more widely adopted for the advantages of low cost and not having to expose the substrate to the high temperature of CNT growth. However, the emission performance from these films is worse than when the film is directly grown on the surface. Although it is possible to pre-fabricate a gate electrode on the substrate before screen print the CNT paste, the gate distance has to be sufficiently large to prevent the conductive paste and CNT from shorting the electrodes. The electron beams generated from both types of the emitter are also highly divergent due to the random directions in which the CNT emit.
Integrally gated CNT field emission electron sources by conventional micro-fabrication have also been reported. Apertures are formed in the gate electrode and aligned to the emitter for a low driving voltage. A bunch of CNTs were then directly grown into the gate hole without further processing. However, there exist large variations among the CNTs, both in the same gate hole as well as from gate hole to gate hole, in their length (thus tip-to-gate distance) and aspect ratios. Such variation, plus the electrostatic screening effect between CNTs in the same gate hole, causes an emission dominated by the ones closer to the gate in each gate hole, and changes the emission threshold field from gate hole to gate hole, resulting in a source of little practical use.
In another type of integrally gated device, there was only one CNT in each gate hole, and the CNTs were at least 5 μm apart. The device still does not address the problem of a varying CNT in its height and aspect ratio in each gate hole and there is little emitter redundancy built in. If one CNT is missing or is not grown properly, which happens often, there would be no emission within 100 μm2. Also, due to the large inter-tube spacing, CNT grow thick, reducing the aspect ratio and therefore increasing the driving voltage.
It should also be pointed out that one of the most common and persistent problems that plagues the fabrication of all types of gated field emitter, whether it is micro-tip or nano-structure type, is the tip-to-gate or cathode-to-gate shorting. To use fully exposed conductive nano-structures as emitters, as are with all the prior art, the uncontrolled length and their fragility can cause serious electrical shorting problems during fabrication and operation. Nano-structures also tend to stick together, to the wall of the gate hole or to the substrate surface upon exposure to moisture or a wet process due to electrostatic force, diminishing their emission performance. Full exposure also subjects the nano-structures to greater ion bombardment during operation, causing faster emitter erosion and re-deposition of the conductive material, and, therefore, shorter lifetime and further possibility of shorting electrodes.